1. Field of Invention
This invention pertains to an apparatus and method for quantitatively measuring low levels of occurrence of certain compounds or substances. More particularly, the invention pertains to detecting and measuring occurrence of highly reactive compounds at levels in the parts per billion (ppb) and parts per trillion (ppt) range.
2. Description of Prior Art
A gas chromatograph (GC) is a powerful analytical device which takes fixed volumes of sample gases, or liquids which can be volatilized, and introduces these samples into a separation column which contains a stationary phase of adsorbent liquid material. The sample is then transported through the column by a mobile phase carrier gas. Individual molecules of the sample gas are adsorbed and then released at different times from the adsorbent stationary phase material in the column. By selection of the proper mobile and stationary phase materials and the temperature program of the GC, a sample containing a mixture of chemical compounds can be separated by the GC in such a way that only one compound will elute from the GC column at a time.
The liquid samples are introduced into the analytical column by two principle agencies: a heated injector (for example, a Grob split/splitless injector) or a cold-on-column injector. The eluting compounds taken from the GC analytical column can be detected by a variety of detectors. Typical detectors are electron capture detectors, electrolytic conductivity detectors, alkali flame ionization detectors, flame photometric detectors, thermal conductivity detectors, flame ionization detectors and mass spectrometers. For all the detectors except the mass spectrometer, the signal peaks generated by the detector indicating presence of specific compounds must be completely separated (resolved) for accurate quantitative analysis to be performed. The mass spectrometer can provide accurate quantitative analysis of co-eluting compounds by specific ion monitoring at a single mass number or simultaneous monitoring of the mass number of several selected fragment ions. The present invention involves a gas chromatograph/ mass spectrometer (GC/MS) system.
In a perfect GC/MS system, every molecule which is volatilized in the injector of the G is detected by the MS. However, because of reversible and nonreversible adsorption and thermal/catalytic decomposition of the volatilized compounds within conventional GC/MS systems, such efficiency is not achievable. This problem increases as the degree of reactivity of the compounds under analysis increases.
A large percentage of conventional GC/MS systems attempt to perform accurate analyses of reactive compounds and use a heated injector of the Grob split/splitless type. Because of the temperature at which they are operated and the internal construction of the injector, this heated injector causes thermal/catalytic decomposition of many highly reactive compounds as described above and is unsuitable for accurate analyses at very low concentrations of the compounds. The conventional high operating temperature of the injector (approximately 10.degree. C. higher than the maximum temperature of the GC oven) creates active thermal sites within the internal elements of the injector and these thermally active sites subsequently induce catalytic and thermal decomposition and reversible/nonreversible adsorption of the reactive compounds under analysis. To prevent these adverse effects caused by maintaining the injector at such high temperatures, a few GC/MS systems use cold-on-column injectors. A conventional cold-on-column injector system consists of an injector body held at room temperature and a 1 to 5 meter length of deactivated fused silica column. The length of deactivated column is known as a precolumn and is used to prevent solvent flooding effects and contamination of the analytical column within the GC. In a conventional system the precolumn resides inside the GC oven and is coupled to the analytical column. However, when the GC oven is cycled from room temperature to approximately 300.degree. C., the heating of the deactivated fused silica precolumn at temperatures exceeding 135.degree. C. cause the precolumn to become thermally activated. The connection between the GC and the MS through an interface is also made with deactivated fused silica tubing which is subject to the same thermal activation problem since conventional systems require the interface also be operated at approximately 10.degree. C. higher than the maximum temperature of the GC oven. Conventional GC/MS systems also require that the ion source of the mass spectrometer be operated at 10.degree. C. higher than the maximum temperature of the GC oven. Under these conditions the metal ionization chamber of the MS also produce catalytic/ thermal decomposition and reversible/nonreversible adsorption of the compounds under analysis.
The present invention avoids these limitations of conventional GC/MS systems by preventing the chemical compounds under analysis, the surfaces of the injector system, the GC/MS interface and the ion source of the mass spectrometer from being subjected to high temperatures in excess of the volatilization temperature of the compound(s) under analysis which cause thermal decomposition and activation. The present invention is designed and operated such that the aforementioned components are operated at a temperature much lower than prescribed by conventional rules and which is programmably specific to the compound(s) under analysis.